Neurophysiological assessment of coma

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Neurophysiological assessment of coma
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Definitions

• Consciousness is the state of awareness of self
  and the environment and coma is its opposite.
  That is the total absence of awareness of self
  and the environment even with external
  stimulation.

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Between these poles there is

• Confusionbewildered but attentive
• delerium..irritable,out of contact but alert
• obtundation.reduced alertness, slowed
  responses
• stupor..responsive only with vigorous and
  repeated stimuli

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Other states of importance

• Vegetative stateThe return of alertness but no
  evidence of cognitive function
• Apallic syndrome..Essentially the same as
  vegetative state
• Akinetic mutismSilent alert immobility, minimal
  motor response to noxious stimulation.
• Locked in syndrome..Evidence of cognitive
  function as distinct from akinetic mutism.

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The state of consciousness reflects

• 1. The level of arousal
• Arousal depends on the integrity of brain stem
  function, in particular the ARAS
• 2. Cognitive function
• Cognitive function predominantly depends on
  cortical and thalamocortical integrity.

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Clinical approach to the comatose patient

• Pathology must involve either
• Bilateral hemispheressupratentorial
• The ARAS.subtentorial
• Diffuse...toxic/metabolic

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Patients can usually be categorised into these
groups

• History
• Assessment of arousal
• pupillary responses
• eye movements
• corneal responses
• breathing pattern
• motor patterns
• deep tendon reflexes

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Supratentorial

• Initiating signs usually of focal cerebral
  dysfunction
• Signs of dysfunction progress rostral to caudal
• Motor signs often asymmetrical

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Fig 19
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Fig 20
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Fig 21
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Fig 22
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Fig 23
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Subtentorial

• History of brain stem dysfunction or sudden onset
  coma
• Oculovestibular features often precede or
  accompany coma
• Cranial nerve palsies present
• Bizarre respiratory patterns common and at onset

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Toxic/Metabolic

• Confusion,delirium,stupor,obtundation precede
  motor signs
• Pupillary responses usually preserved
• motor signs usually symmetrical
• Asterixis,myoclonus,seizures common
• Acid-base imbalance with hypo or hyperventillation

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Psychogenic unresponsiveness

• eyelids actively close
• icewater calorics induce nystagmus
• EEG normal

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The role of EEG in diagnosis

• Differentiates coma from psychogenic
  unresponsiveness
• Identifies non convulsive status epilepticus
• Bilateral delta indicates the patient is either
  deeply asleep or unconscious
• A normal EEG rules out metabolic brain disease as
  a cause of coma.
• A normal EEG in delerium strongly suggests an
  alcohol or drug withdrawal state
• The degree of slowing usually reflects the
  severity of the metabolic encephalopathy

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• In diffuse metabolic encephalopathy the EEG is
  usually more sensitive than the clinical
  assessment with slowing still present when the
  patient has returned to clinical normality.

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The role of EEG in prognosis of anoxic cerebral
injury.

• Five Grades

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Grade 1. Near normal

• Excellent prognosis unless locked inor alpha
  pattern coma

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Grade 2. Theta dominant

• If reactive the prognosis is very good
• If nonreactive survival is usually accompanied by
  neurological sequelae

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Grade 3. Delta dominant

• If reactive the prognosis can be good
• If non-reactive the prognosis is grave provided
  drugs and hypothermia excluded.

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Grade 4. Burst suppression continuous bilateral
periodic sharp waves

• Prognosis grave if drugs and hypothermia excluded
• Often associated with clinical myoclonus.

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Grade 5. Isoelectric

• Prognosis grave if drugs and hypothermia excluded.

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Rare Variants
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Alpha pattern coma

• Anterior predominance
• Unreactive alpha frequency activity.
• Rare survivors but only if brain stem reflexes
  intact.

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Theta pattern coma

• Usually elderly
• 5 Hz theta with low amplitude burst suppression
  morphology
• Grave prognosis

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Spindle coma

• Usually head injury, rarely anoxic injury
• resembles stage II sleep
• prognostically benign.

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The role of EEG in coma prognosis in anoxic injury

• The difficult group are grade II nonreactives and
  grade III. These are also the most common groups.
• SEPs are useful to further define the prognosis
  in these groups.

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The role of EEG in prognosis in severe head injury

• EEG is considerably more limited in
  prognostication in severe head injury.
• Reactivity may be the most useful parameter for
  classifying outcome into good vs bad
• Good being moderately disabled or better
• Bad being worse than moderately disabled.

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Reactivity can be

• Attenuation
• Paradoxical (high amplitude slow waves)
• Doubtful/Uncertain
• absent.
• 90 of patients with preserved reactivity of
  either type have goodoutcomes
• 90 of patients with absent reactivity have bad
  outcomes.
• 20 have uncertain reactivity 70 of
  these have good outcomes.

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Fig 1
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The role of SEPs in anoxic cerebral injury and
severe head injury

• The bilateral absent of the thalamo-cortical
  wave forms (N19, N20, N1)signifies that the
  patient will not recover to better than
  PVS.100 specificity
• However sensitivity is low (20-30 ).
• Hence the interest in the N70

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N70

• Madl et al
• Of 113 patients with a bilateral N70 peak
  latency gt130 msec or absent all but one had a
  poor outcome
• Sensitivity of 94 and specificity of 97
• Sherman et al
• Using a bilateral N70 peak latency gt 176 msec all
  had a poor outcome
• Sensitivity 78 and specificity of 100